According to police, Paris was the passenger in a vehicle that was pulled over outside the Wynn Hotel. The driver was arrested for DUI (drug-related, not alcohol) and when cops searched Hilton, they found her in possession of a controlled substance. Police say tests revealed the substance to be cocaine.
She was arrested and released on her own recognizance.
UPDATE: Paris' boyfriend, Cy Waits, was arrested as well. He was booked on a charge of DUI.
UPDATE: Sgt. John Sheahan told the AP that during the arrest, Hilton asked to go inside the Wynn hotel for privacy. At that time, Sheahan said, Hilton pulled out a tube of lip balm and "a bindle of cocaine in a plastic bag came out of her purse" in plain view of police in the room. Another officer characterized it as a "small amount."